The present invention relates to the field of seismic signal generation. More particularly, it relates to a tool capable of the production of seismic signals, within a borehole, of sufficiently high energy to permit the tool to be used in a deep wellbore.
Many currently available seismic signal generators each have problems which make them unsuitable for use in deep wellbores, i.e., in excess of 4000 feet. Those generators in which the signal is produced on the surface and then transmitted downhole, produce signals that are too weak to permit usage of significant depths. Other generators, such as air guns and perforating guns which locate the source downhole, are capable of producing high energy signals but also produce gas bubbles which create long lasting oscillations in the borehole fluid, thereby inhibiting any sort of medium to high frequency repetition.
The present invention describes a seismic signal source 49 of both the vibrator-type and the impulse-type capable of generating signals of sufficient strength to permit the tool to be used in deep wells for a variety of purposes (e.g., cross-hole seismic measurements, long-spaced and sonic logging tools, downhole lateral seismic systems, and inverted VSPs [Vertical Seismic Profiles]). The invention comprises a downhole signal source including an electric motor and pump that powers a double-headed piston. The pump supplies hydraulic fluid to opposite sides of a first piston head on a piston rod under the control of an electrohydraulic valve. A second piston head (which may be fixed or free-floating) on the same piston rod compresses a fixed amount of hydraulic fluid thereby expanding a flexible bladder which is immersed in wellbore fluid to transmit the desired compressive or impulsive signals to the borehole. The second piston head can compress a fluidic spring (pneumatic or hydraulic) to (a) store energy and increase the magnitude of the signal generated and (b) balance the wellbore fluid pressure which compresses the bladder.
Other features, advantages, and characteristics of the present invention will become apparent after a reading of the following detailed description of the preferred embodiments.